Conventional methods for forming a ferrite film on a solid surface are generally classified into a coating method wherein a binder is employed and a method wherein no binder is employed.
The coating method is usually applied to the production of the ferrite film for a magnetic tape, a magnetic disc and the like, but it has some defects. For example, the magnetic recording density is low because a nonmagnetic binder is present between ferrite particles and therefore it is difficult to apply to an electronic element which is required to be polycrystalline. It is also limited to gamma-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 type ferrite which are generally obtained in needly shape, since shape anisotropy of the ferrite particles is utilized for obtaining magnetic anisotropy of the ferrite film.
The method wherein no binder is employed includes a solution coating method; an electrophoretic electrodeposition method; a dry plating method such as spattering, vaccum deposition and arc discharge; a melt spray method, and a gas-phase growth method. However, these methods require high temperatures (300.degree. C. or more) and cannot use as a substrate a material having a low melting point, low decomposition temperature or poor heat stability.
Therefore, the inventors have proposed a novel method (Japanese Laid-open Patent Application No. 111,929/1982 ) which is quite different in technical concept from the conventional ferrite film forming methods described above. In this method, the surface of a solid is brought in contact with an aqueous solution containing at least ferrous ions, to absorb FeOH.sup.+ or a combination of FeOH.sup.+ and other metal hydroxyoxide ions and the absorbed FeOH.sup.+ is oxidated to conduct ferrite crystallization reaction. This method is hereinafter referred to as a "wet plating method" in contrast to the conventional methods.
The wet plating method, however, has not attained satisfactory rate of formation in the on an industrial scale production. Thus, various improvements have been proposed to obtain a higher rate of formation and better quality of ferrite film.
The formation of the ferrite film proceeds by alternately effecting absorption of the ferrous ions or other metal ions and oxidation of the absorbed ions, as described above. Accordingly, a method has been proposed in which an immersion of a substrate into an aqueous metal ion solution and oxidation, which may be conducted by contacting with a mixed N.sub.2 and O.sub.2 gas, are alternately performed. This method, however, is not practicable because the process is complicated. The method may be considered as conducting the immersion and oxidation simultaneously on the surface of the substrate, but this leads to the production of a ferrite film a poor quality since small particles of ferrite are produced at the same time as the formation of the ferrite film. Great effort must be exerted to remove such ferrite particles.
For solving the above problems, Japanese Laid-open Patent Application No. 179877/1986 proposes that an aqueous solution containing Fe.sup.2+ be sprayed on a rotating substrate. However, since this method is conducted in an oxidizing condition, oxidization is not proceeded uniformly on the substrate and therefore it is difficult to obtain a uniform ferrite film on the substrate.